California Billionaire Tax (Prop 40): Who Pays and When?

California’s ultra-high-net-worth families face a live ballot question this fall. Proposition 40 — the 2026 Billionaire Tax Act — has qualified for the November 3, 2026 ballot and, if voters approve it, would create a one-time wealth tax on residents at the top of the state’s net-worth distribution. The california billionaire tax is uncommon in structure: it uses a January-1 residency snapshot and a December-31 valuation, so an individual who moves out of California after the residency date can still owe the tax if their net worth reaches $1 billion on the valuation date.

At SW Accounting & Consulting Corp, we advise Los Angeles founders, family offices, and closely held business owners. Even before the vote, families near the threshold have real decisions to make on trust funding, gift timing, insurance coverage, and private-company valuation. This post walks through what Proposition 40 actually says, using the initiative text on file with the California Attorney General and other primary California sources.

Who would pay the California billionaire tax? 💰

Individuals with a net worth of at least $1 billion on December 31, 2026 who were California residents on January 1, 2026 — with married couples treated as a single entity.

Under the initiative filed with the California Attorney General, the tax applies to natural persons who meet two tests: California residency on January 1, 2026, and net worth at or above the threshold on December 31, 2026. Married couples are aggregated — the $1 billion floor applies to combined marital net worth, not to each spouse. Residency itself is determined under California’s existing rules, which the California Franchise Tax Board administers today.

Because the residency snapshot is fixed at the beginning of 2026 and the valuation snapshot is at year end, relocating out of California later in the year does not, on its face, avoid the tax. That timing structure is the design choice family offices should focus on first.

How steep is the phase-in and cliff between $1B and $1.1B? 📈

The 5% rate phases in between $1 billion and $1.1 billion, and at $1.1 billion it applies to the entire net worth — producing an approximately $55 million liability at that break point.

The rate mechanics are unusual. Rather than taxing only the excess above $1 billion, the initiative starts the 5% headline rate at $1.1 billion and reduces it by 0.1 percentage points for every $2 million by which net worth falls below $1.1 billion, floored at zero. The result is a steep phase-in band and a hard cliff: relatively small valuation swings inside the phase-in range can move a family’s liability by millions, and at exactly $1.1 billion the tax reaches 5% of the entire net worth — roughly $55 million.

For a founder whose net worth is dominated by illiquid or hard-to-value private stock, that cliff is not academic. A single revised revenue projection or a comparable transaction in the industry can move the reported number across the break point.

How does the california billionaire tax treat trusts and gifts? 🧾

Grantor trusts are pulled into the grantor’s net worth in full; certain non-grantor trusts are attributed at 100% for 2026 transfers and 75% for 2025 transfers; and outright gifts over $1 million after October 15, 2025 are added back.

The trust rules are where much of the planning work sits. A California resident’s net worth includes the full value of any grantor trust — including trusts treated as grantor trusts for federal income-tax purposes and those whose assets would be pulled into the grantor’s gross estate for federal transfer-tax purposes. That reaches many irrevocable trusts, including intentionally defective grantor trusts, that families may not have expected to be counted in a personal wealth-tax base.

Non-grantor, non-exempt trusts trigger a separate attribution rule for threshold purposes: 100% of property transferred to such a trust in 2026 and 75% of property transferred in 2025 is included in the individual’s net worth. Pre-2025 transfers are not clearly addressed, and that ambiguity could matter for whether specific families cross the $1 billion line at all.

On top of attribution, the initiative imposes an independent 5% tax on certain “applicable trusts” — non-grantor, non-tax-exempt trusts that received transfers from an applicable individual or related person — on the trust’s entire net worth, with no separate $1 billion threshold at the trust level. The trustee generally pays, unless the grantor elects to consolidate the trust back into personal net worth.

Charitable structures get a partial carve-out. Trusts exempt from federal income tax under IRC §501 — including §501(c)(3) private foundations and wholly charitable trusts — sit outside the attribution and applicable-trust regimes. Charitable remainder trusts are usually exempt under IRC §664, not §501, so families should not assume a CRT is automatically excluded. A non-grantor charitable lead trust is generally not §501-exempt either and may be treated as an applicable trust.

Outright gifts are addressed separately. Property transferred for less than fair market value after October 15, 2025 — where the property (alone or with substantially interchangeable transferred items) exceeds $1 million in FMV — is added back to the donor’s net worth for tax purposes. In effect, families near the threshold cannot reduce net worth simply by giving assets away before the December 31, 2026 valuation date.

How are private businesses and illiquid assets valued? 🏛️

The initiative overrides several standard valuation techniques — no distressed prices, no minority or marketability discounts, insurance value acts as a floor, and private companies use a book-value-plus-formula figure.

Valuation is where the initiative departs sharpest from familiar practice:

• No distressed or forced-sale pricing — even if the tax itself creates liquidity pressure.
• No minority or marketability discounts — partial interests are valued at a pro rata share of the whole asset.
• Suppressive features may be disregarded — transfer restrictions, shareholder rights plans, and similar mechanisms that reduce appraised value.
• Insurance as a floor — the value cannot be less than the amount for which the asset is insured.
• Private-company formula — FMV equals book value plus 7.5 times average annual book profits, multiplied by ownership percentage.

Complex or illiquid assets typical of family offices — carried interests, restricted stock, unvested equity, cryptocurrency, art, collectibles, contingent interests — are not given specific rules. The general FMV standard applies, which sets up methodology disputes if the tax is enacted.

What should California family offices do before November 3, 2026? ✅

Run a threshold analysis under the initiative’s own valuation rules, inventory trusts and grantor status, review insurance coverage, and stress-test liquidity — regardless of how the vote is expected to go.

Concrete steps we would take before the vote:

• Threshold analysis under initiative rules — model net worth without minority or marketability discounts, using the private-company formula, and with insurance value acting as a floor. Families in the $900M–$1.15B band deserve special attention because of the sharp phase-in cliff.
• Trust inventory — identify grantor vs non-grantor trusts, year of funding, related-person transfers, and trustee knowledge. The consolidation election needs to be evaluated for each applicable trust.
• Insurance review — an over-insured asset carries a taxable-value floor equal to the insured amount. Any reduction in coverage should weigh fiduciary duties, lender requirements, and coverage gaps, not just tax.
• Liquidity planning — the phase-in inside $1B–$1.1B and the full 5% at $1.1B and above can create a tax liability larger than available liquid assets. Evaluate funding sources, borrowing capacity, and asset-sale timing given that forced-sale prices are disregarded.
• Charitable structures — check whether CRTs, CLTs, and split-interest vehicles fall inside the §501 carve-out or the applicable-trust regime.

Proposition 40 at a glance 📊

Frequently Asked Questions ❓

Q. Does the california billionaire tax apply to me if I move out of California in 2026?

The residency snapshot is January 1, 2026. If you were a California resident on that date, moving out later in 2026 does not, by itself, remove you from the tax — you would still be subject to it if your net worth reaches $1 billion on December 31, 2026.

Q. How is the 5% rate calculated between $1 billion and $1.1 billion?

The initiative reduces the 5% rate by 0.1 percentage points for each $2 million by which net worth falls below $1.1 billion, floored at zero. That creates a steep phase-in band inside the $1B–$1.1B range and a full 5% tax on the entire net worth at exactly $1.1 billion.

Q. Are grantor trusts included in my personal net worth?

Yes. The initiative includes the full value of any grantor trust — including trusts treated as grantor trusts for income-tax purposes and trusts whose assets would be pulled into your gross estate for federal transfer-tax purposes. That reaches many irrevocable trusts, including intentionally defective grantor trusts.

Q. Can I reduce my net worth by giving assets away before December 31, 2026?

Not easily. The initiative adds back property transferred for less than fair market value after October 15, 2025 if the FMV — alone or with substantially interchangeable transferred items — exceeds $1 million. In practice, that neutralizes most last-minute gifting strategies for those near the threshold.

Q. Are charitable trusts safe from the tax?

Trusts exempt from federal income tax under IRC §501 — including §501(c)(3) private foundations and wholly charitable trusts — sit outside the attribution and applicable-trust regimes. But charitable remainder trusts are usually exempt under IRC §664, not §501, so families should not assume a CRT is automatically excluded. Non-grantor charitable lead trusts may be treated as applicable trusts.

Q. Where can I read the actual initiative language?

The California Attorney General maintains the official record of qualified initiatives. The California Secretary of State publishes information on ballot measures and voter guide materials for each general election.

Proposition 40 remains uncertain — voter approval, likely constitutional challenges, and regulatory implementation are all unresolved. But the exposure for ultra-high-net-worth California families is significant enough that we recommend a coordinated threshold, trust, valuation, insurance, and liquidity review well before the vote. If you would like a review from a California CPA firm, contact SW Accounting & Consulting Corp. Primary sources: California Attorney General, California Secretary of State — Ballot Measures, California Franchise Tax Board residency rules, and IRC §§ 501 and 664.